Economic damage threshold model for pollen beetles (Meligethes aeneus F.) in spring oilseed rape (Brassica napus L.) crops

The pollen beetle (Meligethes aeneus F.) is the most important pest in Danish oilseed rape (Brassica napus L.) crops. The beetle can cause serious yield losses in both winter and spring oilseed rape crops, and for spring oilseed rape, more than 80% yield reduction can occur. Consequently, in most years, spring oilseed rape crops are treated with pesticides. In the period 1988-1997, 26 spring oilseed rape pesticide field trials were carried out. The pollen beetle infestation was expressed as beetle-days calculated as number of pollen beetles per plant per day and summarised from immigration until end of flowering. Damage caused by pollen beetles was calculated as the yield loss between treated and untreated plots and expressed as a per cent of treated plots. A multiple regression approach was taken, in which the yield loss (YL) was the response variable and beetle-days (BD), day degrees (DD) and precipitation before and after immigration of pollen beetles into the spring oilseed rape fields (P), were the independent variables. The highest value of the regression coefficient and the smallest sum of squared residuals was obtained in a model with only four variables included. The variables were beetle-days, day degrees (DD7b) and precipitation (P7b) in 7 days before the start of immigration and day degrees (DD7a) in 7 days after the start of immigration. The model can be written as YL=0.253BD+0.340DD7b-1.721P7b-0.296DD7a. (r=0.76; p < 0.001; df=139; F=46.02). The economic damage threshold for control of pollen beetles in spring oilseed rape crops can be calculated as 5.0% of the yield. If temperatures are average, the economic damage threshold can be calculated as being from 0.1 to 3.0 pollen beetles per plant depending on the amount of precipitation. Therefore, it can be concluded that the present economic damage thresholds for spring oilseed rape in Denmark of between 0.5 and 1.0 pollen beetle per plant are too crude and must be more differentiated with reference to reducing the application index of insecticides.